1. Field of the Invention
This invention relates to flash devices, and more particularly to flash devices of the type in which the neso-coat for triggering applied on the outer surface of the flash tube is connected to the back metal, forming part of the reflector, and upon application of the trigger signal to the back metal, the flash tube is fired.
2. Description of the Prior Art
In the past, the method of triggering the flash tube in the flash device was to provide the transparent electrode portion of the flash tube with a trigger band to which the trigger voltage is applied. However, the use of this method in view of the efficient production of reflected light necessitates the trigger band be outside the effective part of the reflector. Therefore, the length of the flash tube must be longer than that of the effective part of the reflector, thereby the housing for accommodating the flash tube of the flash device is increased in bulk and size.
The application of such a flash device to so-called stroboscopic built-in cameras, because of the relatively large volume of the light source housing of the flash device, was not advantageous because the camera itself increased in size. Thus, the camera was very inconvenient for the user to carry.
To eliminate these drawbacks, in recent years a new triggering method has been proposed in which without the use of the trigger band, the back metal forming part of the reflector directly applies the trigger signal to the flash tube. By using this method it is possible to reduce the size of the flash tube accommodating housing.
According to this method, the direct application of the trigger signal from the back metal to the flash tube requires that a good electrical connection be established between the back metal and the neso-coat for triggering, applied as a transparent electrode on the outer wall of the flash tube. For this purpose, the flash tube must be pressed against the back metal by a sufficient force. Since the back metal also functions as a reflector it is configured to a circular or parabolic shape. It is difficult to put the flash tube, of which the cross-section is a small circle compared with the back metal, into a prescribed position with high accuracy even when the flash tube rests with stability on the back metal under pressure. This leads to the impossibility of assuring good light distribution and accuracy of each guide number.
In the conventional adjusting method, mention may be made of U.S. Pat. Nos. 3,484,597; 3,127,112 and 4,223,372. In U.S. Pat. No. 3,484,597, the back metal itself is shaped in conformity with the roundness of the flash tube, thus enabling automatic position adjustment of the flash tube. In U.S. Pat. No. 3,127,112, the side walls of the reflector are provided with respective holes upon insertion of the flash tube therein to effect position adjustment of the flash tube. In U.S. Pat. No. 4,223,372, the flash tube is merely positioned in contact with the back metal. Accordingly, the method of U.S. Pat. No. 3,484,597 has a disadvantage that the shape of the back metal cannot be freely designed to suit its action to a reflector, as the circular form of the back metal is necessarily coincident with that of the flash tube. Therefore the light distribution cannot be freely prescribed. Also U.S. Pat. No. 3,127,112 has a similar disadvantage to that of the above-cited patent in that the back metal cannot be constructed in a freely prescribed form. A disadvantage of U.S. Pat. No. 4,223,372 is that the fine adjustment of the flash tube position becomes difficult to perform.
Another requirement of the new triggering method is that because of the direct application of the trigger signal from the back metal to the flash tube, the lead wire for triggering signal transmission is connected to the back metal body itself. However, the back metal is generally made of bright alloy aluminium in view of the impartment of the light reflecting function which sufficiently assures the light distribution characteristics and guide number characteristics of the flash light emitting device. Since, in general, the soldering of an aluminium plate is a very difficult operation, the connection of the triggering lead wire to the back metal is difficult.
In addition to the above-described problems, because the high voltage trigger signal is applied to the back metal, it is essential to establish sufficient electrical insulation of the back metal from the other metallic parts of the flash device. When the flash device using this triggering method is incorporated in a camera with a guide rail for the flash device to pop up for compactness, as the guide rail is positioned near the back metal, sufficient insulation between the guide rail and the back metal must be insured. Otherwise the triggering could not be carried out, and the user would suffer an accident such as electric shock.